1. Field of the Invention
The present invention pertains to methods and apparatus for determining the properties of drilling fluids. Specifically, the present invention pertains to methods and apparatus for real-time measurement and stored data acquisition, evaluation and indication of multiple properties of drilling fluids such as mud temperature, mud weight and API indices of viscosity, i.e. plastic viscosity, yield point, ten-second gel strength, ten-minute gel strength.
2. Brief Description of the Prior Art
Drilling fluids are introduced into the hole of a well during drilling for a number of reasons. One of the main reasons is to prevent loss of control of the well when encountering high pressure gas or other fluid containing formations by maintaining a static pressure head thereon. Drilling fluids also help to maintain a clean hole, protect formations through which the drill string passes, lubricate the drill bit and reduce the friction associated with turning the drill string, etc. Of course, the properties of the drilling fluids are important in considering the effectiveness of the drilling fluids are these uses.
Drilling fluids or "drilling muds" are colloidal suspensions having two parts: a liquid phase and a solid phase. Depending upon the type of fluid, the liquid phase can also have two parts: a continuous phase and a dispersed phase. In the majority of drilling fluids, the continuous phase is water. However, some water-based fluids containing diesel oil will form emulsions of oil droplets (the dispersed phase) surrounded by water (the continuous phase). Muds with a continuous oil phase and a dispersed water phase are called invert emulsions and are classed as oil-based muds along with those fluids without any significant water content.
The solid phase of all drilling fluids consists of commercial and drilled solids. Commercial solids are solids added to the drilling fluids to achieve certain properties. The most commonly employed commercial solids include bentonite, barite, and various thinners. Drill solids are solids that enter the drilling fluids from the well formation in the form of cuttings or carvings.
Rheology has been defined as the science of the flow and deformation of matter. It is important to know the rheological properties of drilling fluids to ensure that optimum properties are maintained for performing their functions. Rheological measurements aid in the diagnosis and treatment of various drilling fluids for related problems encountered during drilling.
The rheology of a drilling fluid includes the measurement of its temperature, weight, and viscosity. The temperature and weight of a drilling fluid appear to be relatively easy to determine. However, the presently used methods, particularly for determining weight, are relatively primitive. The equipment for doing so is usually a mud balance having a balanced beam at one end of which is a mud cup and the other end of which is a fixed counterweight. A rider weight is free to move along a graduated scale on the beam. The cup is filled with mud or drilling fluid to be tested and balanced by moving the rider weight along the graduated scale. One of the problems associated with such measurements is that the mud samples are not always taken from the same location in a mud or drilling fluid tank. Generally, the drilling fluid is heavier toward the bottom of the tank because of the precipitation of solids therein. Thus, different readings can be obtained depending on how far below the surface of the fluid the sample is taken. Furthermore, such sampling is largely by hand and the time from taking the sample until determining mud weight can vary substantially. Thus, this method does not give a real-time measurement or provide for stored data acquisition, evaluation and indication of the multiple properties of drilling fluids. There are other methods of measuring the properties of drilling fluids, i.e. with gamma ray devices and differential pressure devices. However, such methods are relatively expensive and not extremely accurate.
Another rheological property of drilling fluids is viscosity. Viscosity is defined as the internal resistance of a fluid to flow due to an applied force. When a fluid does flow, the internal resistance or force tends to oppose the flow. This internal force is called the shear stress. The internal force can be thought of as a frictional force resulting from one "layer" of fluid as it slides past another. The rate at which these "layers" of fluid within the drilling fluid move past each other is called the shear rate. The ratio of shear stress to shear rate is called the viscosity of the fluid and may be expressed by the formula: ##EQU1##
Drilling fluids can be categorized into two general types primarily based on the viscosity of the fluid: Newtonian fluids (constant viscosity); Non-Newtonian fluids (variable viscosity). The Newtonian fluids are the simplest types of fluids. The shear rate is directly proportional to the shear stress with this type of fluid. Water and many oils are Newtonian fluids. Since viscosity is the ratio of shear stress to shear rate, this means that a plot of shear stress to shear rate would result in a straight line running through the origin. Therefore, for any point along this plot, the shear stress divided by the shear rate will result in the same number.
Non-Newtonian fluids, of which drilling fluids are typical, do not exhibit the direct proportionality between shear stress and shear rate that Newtonian fluids do. The ratio is between shear stress and shear rate is not constant, but is dependent upon the shear rate. Since this ratio is not constant but varies with the shear rate, ratio between shear stress and shear rate is called the apparent viscosity. Thus, a Non-Newtonian fluid is not described by a single viscosity term as with the Newtonian fluid.
Two types of Non-Newtonian fluids are Bingham Plastic fluids and Pseudoplastic fluids. When shear stress is plotted versus shear rate for a Bingham Plastic fluid, a straight line results but does not pass through the origin. Where the line intercepts the ordinate in a plot of shear stress versus shear rate is defined as the yield point (YP). For a Pseudoplastic fluid, a plot of the shear stress versus shear rate has the shape of a curve rather than a straight line.
For a Bingham Plastic fluid, a finite amount of shear stress (y axis intercept) must be applied before fluid begins to flow. This quantity is defined as the yield point (YP). The slope of the line for the Bingham Plastic fluid is called the plastic viscosity (PV). In practical significance, plastic viscosity (PV) is a measurement of the internal resistance to flow due to the amount, type and size of the solids in the drilling fluid or mud. It is due to mechanical friction of the solids in the mud as they come in contact with each other and with the liquid phase of the mud. Solids present in the mud also influence the viscosity property known as the yield point. The yield point (YP) is a measure of the initial resistance to flow due to the electrostatic attractive forces located on or near the surface of particles. It is a dynamic measurement. The yield point is dependent on the type of solids present and their respective surface charges, the concentration of these solids, and the type and concentration of any other ions or salts that may be present.
In practice, as specified by the American Petroleum Institute (API), PV is expressed as the number of dynes per square centimeter of tangential shearing force in excess of the Bingham yield point (YP) that will induce a unit rate of shear. When using a direct reading viscometer, PV is found by subtracting a 300 rpm reading from a 600 rpm reading. Similarly, in practice the Bingham yield point (YP) is determined by subtracting the plastic viscosity (PV) from the 300 rpm reading.
Similar to the yield point (YP), gel strength is a measure of the ability of the mud to form a gel structure when it rests and then become fluid again once agitated. The gel strength is a measure of the stress required to break a gel structure under static (non-flow) conditions. It is also a measure of the same particle to particle forces that is determined by yield point (YP), except that the gel strength is measured under static conditions and the yield point is measured under dynamic conditions. The static conditions used in the measurement of gel strength are for ten seconds and ten minutes at rest.
As previously indicated, the plot of shear stress versus shear rate for a pseudoplastic fluid is a curve rather than a straight line. Mathematically, a pseudoplastic fluid can be expressed as: EQU SS=k(SR).sup.n
where
where n&lt;1 and: PA1 SS=shear strength PA1 SR=shear rate PA1 n=flow behavior index PA1 k=consistency index PA1 .theta.600=dial reading at 600 rpm PA1 .theta.300=dial reading at 300 rpm PA1 n=flow behavior index PA1 .theta.300=dial reading at 300 rpm
The k index is a measure of consistency. The larger the value of k, the more viscous is the fluid. On the other hand, n is the flow behavior index. It is a measure of the degree of non-Newtonian behavior of the fluid. When n equals 1, the fluid is Newtonian. The flow behavior index n is calculated from the equation: ##EQU2## where: n=flow behavior index
Sometimes other dial readings are used to better represent actual mud flow in the annulus. For example, instead of 600 rpm and 300 rpm, dial readings taken from 200 rpm and 100 rpm are used. If such other shear rates are used, the higher sheer rate must be twice the lower shear rate, i.e. 200 rpm/100 rpm, 60 rpm/30 rpm. Once n is determined, the consistency index may be calculated. Unlike n, the k value contains specific units. The equation that is used to calculate k depends upon the units desired. For example, the most common equation used to calculate k is: ##EQU3## where: k=consistency index in units lbs-sec.sup.n /100 ft.sup.2
The most modern method of measuring viscosity utilizes direct indicating viscometers which are rotational type instruments powered by means of electrical motors or hand cranks. Mud is contained in an annular space between two cylinders. The outer cylinder or rotor sleeve is driven at a predetermined constant rotational velocity. The rotation of the rotor sleeve in the mud produces a torque in the inner cylinder or bob. A torsion spring restrains movement. A dial attached to the bob indicates displacement of the bob. Instrument constants are adjusted so that plastic viscosity PV and yield point YP (in the case of Bingham Plastic fluids) or flow behavior index n and consistency index k (in the case of pseudoplastic fluids) are obtained by using readings from the rotor sleeve speeds from 300 and 600 rpm.
There are several problems related to direct indicating viscometers. Of course, with hand crank instruments, rotational speeds are not always the same and the time between the measurements are not always accurate. The temperature of the fluid at the point of measurement may be considerably different than at the point of actual use. There are frictional forces in all these type instruments which can affect the readings thereof. Like in the weight measuring devices of the prior art, the samples placed in these types of instruments may come from different levels in the drilling fluid tank resulting in non-uniform measurements. As in any manual means of measurement, time at the point of measurement may vary considerably so that the measurements taken have no real-time correlation. Thus, it can be understood that the apparatus and methods for measuring the weight and viscosity of drilling fluids are, to say the least, primitive and somewhat less than accurate. Although the inaccuracies may not have been so important in the past, it is becoming more and more important since the cost of drilling fluids is a substantial portion of the total cost of drilling an oil and/or gas well. If the drilling fluids are not of the optimum properties, the cost may be extremely great and the degree of control of the well less than desired.